The present invention relates to a magneto-optical method and apparatus for recording/reproducing data and, more specifically, to such a method and apparatus also enabling data to be directly overwritten.
Recently, a magneto-optical recording/reproducing technique adopting a perpendicularly magnetized film for a recording medium has been widely researched for optically rewritable recording, and a first generation of recording/reproducing apparatus using such a recording medium has already been put into practical use. When data are rewritten in the first generation recording/reproducing apparatus, however, two operations of first erasing already written data and then writing new data are required. It is impossible to realize a so-called direct overwrite, which requires no erasing operation. As a result, the data transfer speed is considerably lower than that of a rigid disk device.
To overcome the above problem, various techniques are now being developed to realize direct overwrite. For example, a technique adopting an exchange coupled magnetic multilayer film as the recording medium is disclosed in Japanese Published Unexamined (Kokai) Patent Appln. No. 62-175948. In this direct overwrite technique, the multilayer film is formed by laminating a first magnetic layer (referred to as a memory layer) and a second magnetic layer (referred to as a reference layer) on a substrate in sequence. The data are recorded by modulating the intensity of a recording laser beam into binary according to recorded data and irradiating the intensity-modulated recording laser beam upon the recording medium. In this recording operation, when the recording laser beam heats the recording medium to a relatively low temperature, magnetic domains of the reference layer that were already initialized prior to this recording operation are transcribed into the memory layer by an exchange coupling force between the two magnetic layers (referred to as L recording). On the other hand, when the recording laser beam heats the recording medium to a relatively high temperature, the magnetic domains of the reference layer are reversed by a recording magnetic field, and then the reversed magnetic domains are transcribed into the memory layer by the exchange coupling force between the two magnetic layers (referred to as H recording). In the reproduction operation, after the reference layer has been initialized, a reproducing laser beam is irradiated upon the recording medium to reproduce so recorded data.
In this technique, therefore, a direct overwrite data recording/reproducing apparatus can be realized for the exchange coupled magnetic multilayer film recording medium by simply attaching a fixed magnetic field generating mechanism for initializing the magnetization direction of the reference layer to a first generation recording/reproducing apparatus having a recording magnetic field and an intensity-modulated laser beam.
In this technique, however, there exists a problem in that already recorded data are destroyed when the intensity of the reproducing laser beam erroneously increases in a reproducing operation or when the ambient temperature or the apparatus temperature rises during the reproducing operation (even at the normal reproducing laser beam intensity). Moreover, this problem increases with increasing sensitivity of the recording medium.